Arsenic

Contents

Arsenic, symbol As, is a rather rare element that by chance has become well known. It is notorious as a poison, but there is more to it than that. Its abundance in the crust of the earth is estimated as 5 x 10-4%, not much more than that of a rare earth metal, and only four times more than that of platinum. However, it very frequently accompanies ores of gold, silver, lead and copper and so becomes an unavoidable by-product. The Boliden gold-copper mine in Sweden from a ton of ore produced 1 oz. gold, 2 oz. silver, 20 lb of copper, but 180 lb of arsenic! This one mine tended to glut the supply of arsenic in the first half of the 20th century. All arsenic comes as a by-product, and ores are not mined specifically for arsenic.

Arsenic is worth studying because of its curious uses, and as an example of an element that stands with one foot in the metals camp, and one foot in the nonmetals camp. Actually, it is erroneous to call an element a "metal" or a "nonmetal" as if this were some inbred quality. An atom is just a certain number of electrons neutralizing the electric charge of a nucleus, and all its properties are only those of the structures in which it takes part. There are no qualities in the atom itself. This is strikingly true in the case of arsenic.

Arsenic has atomic number 33, so it has 33 electrons in the configuration 1s2 2s22p6 3s23p23d10 4s24p3. The same outer shell s2p3 is shared by N, P, Sb and Bi, the two lighter elements above it in the periodic table, and the two heavier elements below. Of these, N and P are evident nonmetals, while Sb and Bi are definitely metals, with metallic lustre and conducting electricity. As falls between, and cannot be classified so simply. The outer shell suggests valences of 3 and 5 for arsenic, and indeed these are observed. The first ionization potential of As is 10.5 V, which is quite high for a metal, but rather low for a nonmetal. The atomic radius of As is 1.25 Å, just a little larger than that of S, 1.06 Å. The single-bond covalent radius is also given as 1.21 Å, and the nonbonding packing radius as 2.0 Å. The only stable isotope has mass number 75. The atomic weight of arsenic is 74.9216 (C12 = 12.0000).

Arsenic by itself likes to form the tetrahedral molecule As4, in which each As is bound to the other three covalently (by sharing electrons). Phosphorus does exactly the same thing. As4 is a yellowish gas that condenses to a yellow, waxy solid with density about 2.0 g/cc. The gas has a characteristic garlic-like odor. This is very much like white phosphorus. However, white phosphorus is reasonably stable, while yellow arsenic is not. In the presence of light, yellow or γ-As rearranges itself to structures of higher density and darker color, such as brown or black β-As of density 4.7. There may be several such structures, but all are nonmetallic, without lustre and not electrically conducting. The most stable form at room temperature, which usually arises spontaneously, is metallic α-As, with grey color and metallic lustre. The lustre soon tarnishes on exposure to air. It is electrically conducting, with resistivity 35 μΩ-cm. Its hardness is 3.5, coefficient of expansion 6.95 x 10-4 per °C, and specific heat 0.0822 cal/g/°C. The substance is definitely a metal, but is brittle and does not melt, but sublimes directly into As4 at 615°C. It is more or less analogous to red phosphorus (which is not a metal, however). We see that arsenic can dress as a metal or a nonmetal, depending on how it aggregates, and how it aggregates depends on the balance between lowest energy and highest entropy (the minimum of the free energy U - TS).

The ancient world knew only two arsenic compounds, but was, of course, unaware of the connection. These were the sulphides realgar, As2S2, and orpiment, As2S3. Realgar was then called sandarach, Greek sandarakh, sandaRAke. It was a soft, light, translucent substance of a deep red-orange colour that was ground and used as a pigment. The other was a soft, light, translucent substance of a fine yellow colour, also used as a pigment. In Greek, it was also called sandarach or arsenikon, "arsenikon." The Greek word arsen, "arsen," means "male," or by extension "strong." How this came to be associated with the compound is not known. In Latin, it was called auripigmentum, from which the French or-piment directly comes. Gaius Caesar (Caligula) engaged in a little alchemy to see if he couldn't coax the yellow color onto some heavy metal and get gold, but he was not successful. This was long before mystical alchemy was introduced from the Arabs, and should be classed as a chemical endeavour. The toxic nature of arsenic was unknown to the ancients.

Realgar and orpiment are rare and rather unusual arsenic minerals. They are secondary alteration products found in association with native metallic arsenic. In them, arsenic acts like a metal, but the compounds are mainly covalent. The structure of realgar is something like S=As-As=S, though the simple formula AsS gives the relative amounts of As and S. Phosphorus, likewise, forms a number of sulphides. They clearly came to notice because of their bright colours; the obscure, grey metallic arsenic would have been overlooked, perhaps considered an impure lead. Native arsenic is rather rare, but is found in a number of places, such as the Harz and Erzgebirge mountains.

In addition to this, the Egyptians used arsenical copper, probably for mirrors, which was called pseudargyros. Zosimos of Panopolis, an Alexandrian alchemist of the 2nd century BCE, is said to have made metallic arsenic from realgar. The property of mixtures with slaked lime to make hair fall out was also noted, and has been used in the Orient instead of razors for shaving, a seemingly perilous procedure.

When arsenic compounds are heated under oxidizing conditions (as in air) the oxide As2O3, arsenious oxide, is often produced. This is a volatile substance appearing as a dirty white vapour smelling of garlic and condensing as a white powder, or forming small octahedral crystals. Strictly speaking, the name arsenic applies to this compound, not to the metal. In German, the metal is called Arsen, the oxide Arsenik, but in English "arsenic" is used for both. This is by far the most common way that arsenic appears. It is the most common way arsenic is produced and sold at present, and is usually called "white arsenic." White arsenic was described by the Arabic alchemist Geber (Jabir ibn-Hayyan, 760-815) in the 8th century, but he took it for a form of sulphur driven out when metallic arsenic was heated. A little later, the Arab iatrochemist Avicenna (Ibn-Sina, 979-1037) recognized the toxic properties of white arsenic. Kunkel later showed that white arsenic did not contain sulphur. When arsenic is added to copper, the copper is decolorized and becomes white. In ancient times, this was called pseudargyros, and was taken to be silver by later alchemists. This alloy was popular with counterfeiters after it became known to them. Albertus Magnus, the Renaissance alchemist, showed that by further heating the arsenic could be driven out and the copper restored. He supposed, quite rightly, that white copper was a mixture of copper and arsenic, and that arsenic was a metal common to several important compounds. Some say that he heated white arsenic with soot and obtained metallic arsenic around 1250. Others say that the pure metal was first isolated by Johann Schroeder in 1694, or that it was first definitely prepared in 1641. Albertus probably deserves some credit for recognizing the identity of arsenic, if not of actually making it synthetically. Arsenic is, after all, present in nature in metallic form. Arsenic metal is easily made by heating the oxide with carbon (coke or charcoal) in a retort, and condensing the vapors. It is about ten times more expensive than white arsenic, weight for weight.

In the trioxide As2O3, As has valence +3, while in the pentoxide As2O5 the valence is +5. When oxides such as these are dissolved in water, they attract H+ and OH- ions and may rearrange their structures. Now, if we take Na2O, the oxide of the clearly metallic sodium, adding water results in Na2O + 2H2O → 2NaOH → 2Na+ + 2OH-. The solution becomes basic because of the abundant OH- ions. In the case of As2O5, we find that As2O5 + 3H2O → 2H3AsO4. The molecule formed is orthoarsenic acid, which dissociates to give H+ ions, and an acidic solution. Recall the similar orthophosphoric acid, H3PO4. Here, arsenic behaves as a nonmetal, like phosphorus, and can form salts with metals called arsenates. Similarly, As2O3 hydrolyzes to H3AsO3, arsenious acid, whose salts are the arsenites. The arsenic acids cannot be isolated in stable dry form (orthoarsenic acid seems to form a hydrated crystal); on evaporation of the solutions, the oxides are reformed. The arsenate ion, AsO4---, is very similar to the phosphate ion, PO4---. Arsenic is probably poisonous because the arsenate attempts to replace phosphate in metabolic processes, and fails miserably.

A few arsenate minerals are found, mostly in altered surface deposits and even deposited by hot springs. Mimetite, Pb5Cl(AsO3)3, is a soft, heavy mineral found in supergene-enriched zones. Scorodite, FeAsO4·2H2O (ferric arsenate) is soft and light, often in hot springs. Erythrite, Co3(AsO4)2·8H2O (cobaltous arsenate) is a bright red mineral called cobalt bloom. There is a series of such minerals in which Ni replaces Co, ending with annabergite or nickel bloom. The water of crystallization should be noted. These are all rare. A hot spring with abundant arsenic would, of course, be poisonous, and such springs have been known.

In most arsenic minerals, the arsenic replaces sulphur in more familiar minerals. The hard and heavy mineral NiAs, niccolite or kupfernickel, is an example. Silver white cobaltite, CoAsS, is hard and heavy and occurs in pyritohedrons, but is distinguished from pyrite by its colour. FeAsS, arsenopyrite, is perhaps the commonest arsenic mineral, and is also called mispickel. It is found in striated prisms. The mineral once known as smaltite, now as skutterudite, is (Co,Ni,Fe)As3; it contains varying quantites of the iron group metals. Proustite, Ag3AsS3, or "ruby silver," is soft and of medium weight. Cu3AsS4 is black enargite, the source of arsenic in the mines of Butte, Montana. Note that in the last two compounds, S has replaced O in the arsenate and arsenite. Enargite is cuprous thioarsenate.

Arsenic forms a number of halogen compounds with valence +3 and +5. For example, AsCl3, arsenious chloride or "butter of arsenic," an oily liquid that decomposes when added to water; and AsF5, arsenic fluoride, a colourless gas. The "ic" denoting the higher valence state conflicts somewhat with the attributive "ic" in the name. There is also As2S5, arsenic (arsenicic) sulphide. These compounds are not found in nature.

The gas AsH3 or arsine is analogous to NH3, ammmonia, and PH3, phosphine. This compound was discovered by Scheele in 1775. It is highly toxic, which was brought out forcefully soon after its discovery by the death of the prominent chemist Gehlen of Munich during experiments with it. In arsine, the conventional valence of arsenic is -3. If arsine is produced somehow, strong heating will decompose it into arsenic and hydrogen, and the arsenic vapor will condense as a shiny film of metallic arsenic, the "arsenic mirror," at a colder place. This behaviour is used in the Marsh test for arsenic. The material to be tested is put in a flask with granulated zinc and hydrochloric acid is added. Any arsenic is changed to arsine as hydrogen is evolved and passes off in the evolved gas. The tube carrying the hydrogen is strongly heated at a certain point. If arsenic is present, an arsenic mirror is formed on the cooler part of the tube farther on. The hydrogen can also be lighted at the end of the tube. A cold surface held above the flame will also condense any arsenic that is formed. Incidentally, hydrogen flames contain no carbon, and so are quite invisible, even in a dark room. Unless you are very careful, these flames can be dangerous. Arsine generally oxidizes readily in the atmosphere.

A closed flask containing some arsenic can divert the experimenter. If the arsenic deposit is heated, it sublimes and condenses at some cooler part of the flask. The arsenic can be chased around the flask in this way with a flame.

Arsenic, despite its reputation, had little application as a war gas. Some derivatives of arsine, such as methyldichloroarsine, have been used to some degree, the arsenic giving them a poisonous action as well as the more immediate and useful effects of a vesicant, lung irritant or sternutator. Arsine itself would be quite useless, because of its short life in air. The frightening French Vincennite, one of the first poison gases used in World War I, contained not only hydrogen cyanide, but also arsenic trichloride. It was just an ill-founded mixture of horrors, and was spectacularly ineffective.

The Marsh test was important in forensic investigations, because of the most famous property of arsenic, its extreme toxicity. The 50% lethal dose is about 70 mg. This is probably similar to the high toxicity of white phosphorus, but it occurs also with all soluble arsenic compounds, while phosphorus compounds are generally nontoxic. Arsenic could be administered in small doses that added up to a fatal dose, causing a mysterious death. These arsenic compounds are practically tasteless, which made their administration easy. Arsenic does not appear to be popular with today's murderers and assassins, but it was once very accessible in insecticides and rat poisons, or from your parish alchemist. Some isolated communities, such as the "arsenophagoi" of southern Austria, made a practice of regularly consuming small amounts of arsenic, so that their bodies acquired a tolerance for it, and they could consume amounts that would normally be fatal. They thought that this made them "tougher," but this was probably an illusion, since they were no less susceptible to injected arsenic (how this was determined I cannot imagine). Not much can be done as first aid in cases of arsenic poisoning. Chemical antidotes are very doubtfully effective. Emetics (soapy water) and cathartics (Epsom salts) could probably help, or else spread the poison around. Oxygen and rest are best for arsine poisoning.

For many years, the greatest use of arsenic was for poisons, and in particular, insecticides. Lead arsenate, Pb3(AsO4)2 was a common insecticide; later, the calcium salt replaced it. Paris Green, copper acetoarsenite, Cu(C2H3O2)2 · 3CuOAsO3, and copper arsenite (Scheele's Green) CuHAsO3 are also useful. These are all insoluble compounds (if they were soluble, they would just wash off or out in the rain, besides being very poisonous to things other than insects). The last is used as a wood preservative, giving a characteristic green tint. Chemophobes attack any such use, but it is a very small hazard indeed. They are frightened by the name "arsenic" however it appears. Arsenic is not a heavy-metal poison, but more or less like copper. It disperses rapidly in the environment. Of course, anything more than traces should be carefully avoided. There was a so-called Fliegenstein, a condensed white arsenic that killed flies that landed on it, and an arsenic soap for preventing the growth of larvae in animal hides that were to be stuffed.

The boll weevil, Anthonomus grandis, moving northward, attacked Texas cotton crops as early as 1894, and by 1909 had spread to all the Gulf states. One fairly effective method of control was dusting the cotton, preferably at night, with calcium arsenate. Arsenic is not an essential plant nutrient.

The first syphilis plague in Europe occurred in 1495, not long after Columbus and his sailors had returned from America, where they had traded smallpox for this great pox. When it broke out while the French were sacking Naples, the Italians called it the French Disease, while the French called it the Naples Disease. It was named after a shepherd Syphilis in a play, whom it pleased God to infect, but was soon known in polite company by euphemisms. It is caused by the spirochete treponema pallidum, and attacks over many years in three stages, each worse than the last, though sometimes missing the tertiary stage. There was no effective treatment, though many were tried.

Paul Ehrlich (1854-1915), the bacteriologist, received the Nobel Prize in Medicine for 1908. In 1909 he announced the arsenic compound that was known as Salvarsan, or as "606" after its number in a list of preparations. This was phenol to which an arsenic atom had been added opposite to the OH, and an amine group, NH2, next to the OH, whose systematic name was arsphenamine. It was usually accompanied by HCl to make it more soluble. This molecule is small enough to get past the intestines and into the blood stream, where it played havoc with treponema. Because it was also quite toxic, it had to be administered in small doses over a considerable period, which made therapy inconvenient. It was, however, quite effective and relieved great amounts of suffering. Salvarsan was the first synthetic chemotherapeutic agent. Around 1943 it was finally superseded by penicillin. Penicillin was just as effective, and could be administered in large doses, shortening the time of treatment.

White arsenic can be added to glass melts in a concentration of about 0.5% to remove the objectionable green tint produced by iron impurities. Metallic arsenic also has a few uses as an alloying element. Arsenic hardens lead, and is used in alloys for making lead shot and in lead-acid battery plates. It is also said to make lead shot more spherical, when made the old way in a shot tower. Arsenic has long been known to decolorize copper, to the delight of counterfeiters. Smaller amounts of arsenic raise the annealing temperature of copper, so that copper intended to be toughened by work hardening does not be come soft when exposed to heat. This was important in the fabrication of copper locomotive fireboxes, long used in Britain from a distrust of steel in this application. Arsenical copper is strong and very tough. Arsenic does destroy the conductivity of copper, so if a tough and still highly conductive copper is required, cadmium is used in place of arsenic.

Arsenic sulphide, As2S2 is used in pyrotechnic mixtures to produce a brilliant white light. It can be produced in a transparent form called ruby sulphur. Paris green makes a blue light, but that is due to the copper, not the arsenic.